Aircraft: WIDEBODY TRANSPORT
Phase of flight: Taxi
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Person / Animal / Bird

We landed in PAE and taxied to parking uneventfully. After we had parked a member of the ground crew came on the headset and welcomed us as we were waiting for ground power to be plugged into the aircraft. After the ground power was plugged in the CA (Captain) shut down the number X engine and the ground crew member who was on the headset confirmed we had a good shutdown on the engine. At no time did either myself or the CA hear the ground crew say anything about not releasing the parking brake. The CA did not release the parking brake at this point because we had not yet received confirmation that the chocks were in place. We completed the shutdown checklist; and after the R1 door was open we sent the postflight report and performed the secure checklist. After this I got up; packed my bag; and went to the bathroom. While I was in the lavatory; I heard a member of the ground crew come up and talk with the Captain. Because of the location of the lavatory I was able to hear the conversation between the CA and the ground crew member. I heard the CA ask if the chocks were in place and heard the ground crew member confirm that the chocks were in; I did not hear the ground crew member say anything about not releasing the brakes. I heard the CA release the brakes; it felt like the aircraft moved slightly; but not much. Although that was just my perception; since I couldn't see outside. After I came out of the lavatory; I continued getting ready to leave the aircraft while the CA went into the lavatory. While I was in the flight deck; I heard a member of the ground crew on the headset asking us if we meant to release the brakes. I grabbed the hand mic and told him that yes; we were told the chocks were in so we released brakes in accordance to standard procedures. He told me that we were not supposed to release the brakes at this parking spot and that a member of the ground crew had been near the gear when the brakes were released. I informed him that there was nothing in our manuals about leaving the brakes set. He seemed to think there was something in our manuals about not releasing the brakes. I told him that I would alert the CA to the issue. When the CA came out of the lavatory I told him what was going on and we decided that we would just talk to the ground crew when we got downstairs. I got down to the ramp after the CA; when I arrived at the bottom of the stairs a member of the ground crew was there and was quite upset. He told us we had nearly run him over by releasing the parking brakes and kept saying that we should have known not to release the brakes. He was clearly quite upset with us. Members of the ground crew kept mentioning a manual that I think they called the [name] ramp manual; or something similar to that. We calmly informed the ground crew that we didn't have that manual and that our company information pages contained no information about leaving the parking brake set. We were also asked by the ground crew if we were told on the initial headset contact to leave the parking brake set. We told the ground crew that no one told us that; although they didn't seem to believe us. After a couple of minutes another member of the ground crew escorted us out to the limo and we left. As a side note; neither myself or the CA are based in PAE and we don't fly Aircraft X very often. We did review the company pages prior to arrival and we checked them again after our arrival. Nowhere in the company pages could we find instructions regarding leaving the parking brake set. We would never have released the parking brake if we had been told not to. The company pages in our Jepp manuals should be updated to warn crews not to release the parking brake unless specifically cleared to. The local ground crew should be reminded to verbally warn crews not to release the parking brake as this differs from our standard procedure. Air carrier First Officer reported the aircraft possibly moved after the Captain released brakes and the ensuing ramp safety issues this created.